1. Field of the Invention
The present invention relates to a dielectric breakdown prediction apparatus and method, and to a dielectric breakdown life-time prediction apparatus and method. More particularly, the present invention relates to a time dependent dielectric breakdown (TDDB) prediction method using a method for predicting dielectric breakdown, used for semiconductor devices, and a step stress method.
2. Description of the Related Art
FIG. 9 illustrates typical I-V characteristics based on a conventional dielectric breakdown prediction method when a voltage is applied to an electrode in a condition in which an insulating film is sandwiched by the electrode. In the conventional dielectric breakdown prediction method, a voltage V.sub.BD is regarded as a breakdown voltage when an observed electric current has reached a reference current I.sub.cr.
However, insulating films such as thermally oxidized films are classified into three types of failure modes: A mode, B mode and C mode, according to defects within the insulating film. The I-V characteristics of the insulating film in these three types of failure modes are as indicated by curve 2, curve 3, and curve 1, as shown in FIG. 10. At this time, since only voltage V.sub.BD for the reference current I.sub.cr can be checked according to the conventional dielectric breakdown prediction method, breakdown voltage V.sub.C,1 of curve 1 will not be correct though it is possible to accurately check breakdown voltage V.sub.A,1 and V.sub.B,1 of curves 2 and 3, respectively, by I.sub.cr,1 of FIG. 10. In this connection, if the breakdown voltage is checked by reference current I.sub.cr,2 in order to accurately determine the V.sub.C,2 of curve 1, voltages V.sub.A,2 and V.sub.B,2 are not accurate at this time.
Since, as described above, the breakdown voltage is checked using only one predetermined current I.sub.cr in the conventional dielectric breakdown prediction method, it is impossible to determine an accurate breakdown voltage for an insulating film having various types of failure modes.
Use of the dielectric breakdown prediction method permits TDDB characteristics based on a step stress method to be predicted, and thus there is an advantage in that the reliability of insulating films can be predicted earlier. Accordingly, in the conventional step stress TDDB prediction method, as shown in FIG. 11, t.sub.BD at an electric field E.sub.G is estimated from the relation of the dielectric breakdown life-time t.sub.BD .varies.exp(-.beta..multidot.E.sub.G) by the following equation (4): ##EQU1## However, when the breakdown mechanism of a thermally oxidized film was checked in detail, it was found to conform to the reference of "IEEE Trans. Electron Devices ED-32, P413(1985), I. C. Chen, S. E. Holland, and C. Hu".
Since the TDDB prediction method obtained by equation (4) uses the relation of the dielectric breakdown life-time t.sub.BD .varies.exp(-.beta..multidot.E.sub.G), it is impossible to accurately obtain TDDB characteristics.